1. Field of the Invention
The present invention relates to a battery charging device in general, and relates in particular to a battery charging device for charging a sealed alkaline battery for use with transport vehicles having no driver.
2. Technical Background
In recent years, driverless transport vehicles have become popular for use in clean rooms for producing semiconductor integrated devices. Such driverless transport vehicles are powered by open-type alkaline batteries capable of being fast charged. Recently, an automatic charging apparatus having no driver is used to recharge these alkaline batteries.
FIG. 2 illustrates one such battery charging apparatus. In this figure, the reference numeral 1 refers to a driverless transport vehicle; 2, an alkaline battery provided in the vehicle; 3, a receiving coupling electrically connected to the alkaline battery 2; 4, a controller; 5, an electro-optical data communication device electrically connected to the controller 4 for data communication with outside environment.
The reference numeral 6 refers to a recharging station. In the recharging station 6, 7 refers to a power supply coupler which is constructed so as to be freely movable in the direction indicated by the arrow A, and is electrically connected to the receiving coupler 3 of the driverless transport vehicle 1. The reference numeral 8 refers to an electro-optical data communication device of the recharging station 6 to communicate with the electro-optical data communication device 5 of the driverless transport vehicle 1 (shortened to vehicle 1 hereinbelow). The reference numeral 9 refers to a power source and is electrically connected with the power supply coupler 7 and the optical data communication device With this configuration of the apparatus, when the vehicle 1 arrives at the charging station 6, the power supply coupler 7 moves towards the vehicle 1 guided by the data communication devices 5, 8, and connects with the receiving coupler 3. Following this event, the charging device 9 begins charging the alkaline battery 2. During the charging period, the controller 4 monitors the output voltage of the battery 2 periodically and the data indicating the monitored voltage are transmitted to the charging device 9 via data communication devices 5, 8.
FIG. 3 is an illustration to explain the charging process. The solid line L1 shows the time-dependence of the charging current to the battery 2, and the solid line L2 shows the variations in the voltage at the terminals of the battery 2. As shown in this figure, the charging device 9 performs charging at a constant charging current. Next, after the voltage at the terminals of the battery 2 has reached a specific voltage V1, the charging device 9 performs charging at a constant voltage. When the charging current reaches a certain current A2 (or after a fixed interval of time), the charging operation is ended.
In the clean room environment, in order to improve manufacturing efficiency, alkaline batteries are used, since such batteries can be rapidly charged.
Occasionally, however, over-charging of the battery occurs when the normal voltage for full charge cannot be obtained for some reason or when the charging device malfunctions. Unfortunately, gasses from the alkaline battery can negatively influence the environment of the clean room. Thus, a sealed alkaline battery has been used in place of the open-type alkaline battery. The open-type alkaline batteries are provided with a pressure release safety valve for venting the excess battery interior gases (hydrogen and oxygen) caused by over-charging. The sealed alkaline battery has a safety valve, and when the interior pressure in the battery cell reaches a certain value, the safety valve is opened, thereby spreading gasses in the clean room. In clean room operations, such release of gases and alkaline vapors will have a damaging effect on the products (integrated circuits and others), and adversely affect the yield. Therefore, it is necessary that such over-charging events be strictly controlled.